Metallocene catalysts allow the production of polyolefins with unique properties such as narrow molecular weight distributions and narrow chemical compositions. These properties in turn result in improved structural performance in products made with the polymers, such as greater impact strength and clarity in films. While metallocene catalysts have yielded polymers with improved characteristics, they have presented drawbacks when used in traditional polymerization systems. For instance, when metallocene catalysts are used in fluidized bed reactors, drawbacks include sheeting and the related phenomena drooling.
Various methods for controlling sheeting have been developed. Such methods typically involve monitoring the static charges near the reactor wall in regions where sheeting is known to develop and introducing a static control agent into the reactor when the static levels fall outside a predetermined range. A positive charge generating additive is used if the static charge is negative, and a negative charge generating additive is used if the static charge is positive. Drawbacks to such monitoring methods include inefficiencies when used with metallocene catalysts.
Further drawbacks to metallocene catalysts when used with polymerization systems include reactor discontinuity problems. Techniques have been developed to overcome such drawbacks such as coating the polymerization equipment, controlling the polymerization rate, particularly on start-up, and reconfiguring the reactor design and injecting various agents into the reactor. Such techniques also have various drawbacks such as inefficiencies.
Continuity additives have been developed to overcome such deficiencies. However, conventional continuity additives also have drawbacks. Such drawbacks may include reduced catalyst productivity.
Consequently, there is a need for improved methods for improving polymerization processes. There are also needs for improving catalyst and reactor performance.